The present invention relates to a lead frame flash removing method and apparatus for removing a flash formed when molding a resin portion integrally with a lead frame.
Ultra-high frequency devices include a transistor, an IC, an optical element, a surface acoustic wave element, a resonator, and the like. In such an ultra-high frequency device, as the package becomes large, the inductance component of the lead is increased, and the loss in a high frequency band is increased. Therefore, the ultra-high frequency device must be stored in a very small package having a diameter of about 2 mm. Conventionally, the ultra-high frequency device is incorporated in a hollow ceramic package. As the ceramic package is expensive, the ultra-high frequency device tends to be incorporated in a resin mold package.
Conventionally, when a package is molded from a resin, after molding, a flash is formed on the lead frame. To remove this flash, the following methods are available, e.g., a honing method of spraying an abrasive agent mixed in an air flow or water flow, and an electrolytic method of dipping the lead frame in an alkali solution as an electrode, and performing electrolytic process, so that bubbles are generated from the lead frame, thereby floating the flash from the surface of the lead frame.
In the conventional flash removing methods described above, a problem occurs when a relatively thick flash is formed. More specifically, when removing a thick flash by the honing method, since the spraying pressure must be increased, the surface of the molded portion is undesirably ground with the abrasive agent. Also, the adhesive properties between the lead frame and the mold are decreased, or the mold case may be undesirably disconnected from the lead frame. When removing a thick flash by the electrolytic method, since the electrolytic solution is difficult to permeate between the flash and the lead frame, a time required by the electrolytic process becomes very long, thus decreasing the efficiency.
In a method of this type, since the processing efficiency is a very significant factor, it is not advantageous to remove all the flashes from the lead frame by the electrolytic process, and it is better to remove flashes remaining after the electrolytic process by another scheme. However, since the flashes remaining after the electrolytic process often firmly adhere to the lead frame, they cannot be removed easily. For example, as shown in Japanese Patent Laid-Open No. 4-96238, when a method of blowing off the flashes remaining after the electrolytic process is employed, the water pressure must be very high.
A conventional flash removing method of removing a flash by the electrolytic method will be described with reference to FIGS. 4 to 10. FIGS. 4 to 10 show the respective steps until an RF device is formed from a lead frame. Referring to FIG. 4, a lead frame 1 is used to manufacture an RF device. An island 2 for mounting a chip (to be described later) thereon, inner leads 3 to be wire-bonded and connected to the chip electrodes, and outer leads 4 extending to the outside are integrally formed in the lead frame 1.
Referring to FIG. 5, reference numeral 5 denotes a molding die used for molding the case portion of a hollow package on the lead frame 1. The molding die 5 is constituted by a lower die 6 supporting the lead frame 1, and an upper die 7 cooperating with the lower die 6 to sandwich the lead frame 1. A cavity 8 of the molding die 5 is formed by forming a recessed portion 6a having a square opening in the upper surface of the lower die 6 and by forming a recessed portion 7a defining a square opening in the lower surface of the upper die 7. A molding resin flows into the cavity 8 through a runner 6b and a gate 6c formed in the lower die 6.
Referring to FIG. 6, reference numeral 9 denotes the case of a hollow package molded integrally with the lead frame 1. Reference numerals 16 and 17 denote flashes generated to attach to the surface of the lead frame 1. Referring to FIG. 7, an electrolytic process carbon electrode 12 connected to the anode of a power supply, and the lead frame 1 connected to the cathode of the power supply are dipped, together with the package 9, in an electrolytic solution 11 stored in an electrolytic cell 10. In the flash removing method described in Japanese Patent Laid-Open No. 4-96238, a 5% aqueous NaOH solution is used as the electrolytic solution 11, and is heated to 50.degree. C.
Referring to FIG. 8, reference numerals 13 denote nozzles for injecting water 13a. Referring to FIG. 9, reference numeral 18 denotes an RF chip; and 19, bonding wires. Referring to FIG. 10, reference numeral 14 denotes the cap of a hollow package. The cap 14 is adhered, with an adhesive material 15, to the upper surface of the package 9 formed by molding.
The steps in manufacturing an RF device by using the lead frame 1 shown in FIG. 4 will be described. First, as shown in FIG. 5, the lead frame 1 is inserted in the molding die 5 to perform molding. Molding is performed by flowing a molten epoxy resin from the runner 6b into the cavity 8 through the gate 6c and solidifying it as it fills the cavity 8. The liquid epoxy resin flows into the recessed portion 7a of the upper die 7 of the cavity 8 through the gap between the inner and outer leads 3 and 4 of the lead frame 1.
The island 2 and the inner leads 3 of the lead frame 1 are not supported by the lower die 6, but are supported by their own rigidity at positions where they come in contact with the lower surface of the upper die 7. Thus, the liquid epoxy resin also enters the gap between the island 2 and the upper die 7, and between the inner leads 3 and the upper die 7. After the entered epoxy resin solidifies, it forms thick flashes to remain. The liquid epoxy resin enters, although slightly, also portions around the cavity 8 and between the lead frame 1 and the upper die 7 and between the lead frame 1 and the lower die 6.
After molding, the lead frame 1 is extracted from the molding die 5, as shown in FIG. 6, thereby obtaining the lead frame 1 molded integrally with the case 9. A square frame portion 9a is formed, to be integral with this case 9, from the epoxy resin solidified in the recessed portion 7a of the upper die 7. The island 2 and part of the inner leads 3 are exposed to the bottom surface of the recessed portion surrounded by the frame portion 9a as they are covered with the flashes 16. The flashes 17 also attach to the upper and lower surfaces of the proximal end portions of the outer leads 4 projecting from the case 9.
The lead frame 1 is dipped in the electrolytic cell 10, as shown in FIG. 7, and a DC voltage is applied across the lead frame 1 and the carbon electrode 12, thereby electrolytically processing the lead frame 1. At this time, the lead frame 1 is set as the cathode. When the electrolytic process is performed in this manner, hydrogen bubbles are generated from the lead frame 1 to form gaps between the lead frame 1 and the flashes 16, and between the lead frame 1 and the flashes 17. As a result, part of the flashes 16 and 17 are separated from the lead frame 1.
After the electrolytic process, the water 13a is injected to the upper and lower surfaces of the lead frame 1 from the nozzles 13, as shown in FIG. 8. By this water injection, flashes 16a and 17a remaining after the electrolytic process are blown off from the lead frame 1 and are thus removed. The flashes 16a and 17a remaining after the electrolytic process firmly attach to the lead frame 1. To remove the flashes 16a and 17a completely, the pressure of injected water is set to 300 kg/cm.sup.2 in the method shown in Japanese Patent Laid-Open No. 4-96238.
Thereafter, as shown in FIG. 9, the RF chip 18 is bonded to the island 2 of the lead frame 1, and the electrodes (not shown) of the RF chip 18 and the inner leads 3 are electrically connected through the bonding wires 19.
The flashes 16 and 17 must be completely removed from the island 2 and the inner leads 3 in the water injecting step described above so that the flashes 16 and 17 will not remain on them. If the flashes 16 and 17 remain on the island 2, the resistance between the ultra-high frequency device, e.g., the chip 18, and the island 2 is increased, thereby degrading the electrical characteristics. If the flashes 16 and 17b remain on the inner leads 3, the bonding strength is decreased, thus degrading the reliability.
After bonding, as shown in FIG. 10, the cap 14 is adhered to the frame portion 9a of the case 9, the outer leads 4 are cut at predetermined positions, and the resultant structure is subjected to molding, thereby manufacturing the RF device in which the RF chip 18 is hermetically sealed with the case 9 and the cap 14.
As described above, in the conventional flash removing method, in removing the flashes 16 and 17 formed on the surface of the lead frame 1, if all the flashes 16a and 17a remaining after the electrolytic process are to be blown off with the water pressure, the water to be injected must be pressurized to a high pressure of about 300 kg/cm.sup.2. Therefore, a large-sized pressurizing unit for obtaining a high water pressure becomes necessary, and the cost of flash removing is thus increased.
Conventionally, since the package is large, no problem particularly occurs even if high-pressure water is injected. However, it is difficult to apply the same method to an ultra-high frequency package. More specifically, an ultra-high frequency package is hollow and very small, and its case 9 and lead frame 1 are connected to each other with only a resin portion of about 0.4 mm. Accordingly, if high-pressure water is injected to such an ultra-high frequency package, the case 9 may be blown off from the lead frame 1 with the water pressure, or the resin adhesive properties between the lead frame 1 and the case 9 are decreased, leading to a decrease in yield and reliability.
Still another flash removing method employing dry blasting is disclosed in Japanese Patent Laid-Open No. 5-335434. According to this method, an alumina grinding material is sprayed to the flash for 5 seconds to perform sand blasting. With this method, the damage to the case and the lead frames is large to satinize their surfaces. Then, when a chip is mounted, the obtained electrical characteristics are degraded. Since a solid matter is directly used as the abrasive agent, it clogs the pipe, and a process unit for collecting dust after spraying becomes necessary.